The prior art shows electrostatic copying machines of the developed image transfer type in which the surface of a photoconductor is successively moved past a charging station at which the surface receives a uniform electrostatic charge, past an exposure station at which the charged surface is exposed to a light image of the original to be copied to form an electrostatic image, past a developing station at which the electrostatic image is developed by particles of toner, past a transfer station at which the developed image is transferred to a sheet material, such as plain paper, and then to a pickoff station at which the sheet carrying the developed image is removed from the surface of the photoconductor and delivered by rollers to a delivery tray. In such machines, it is desirable that the user be able selectively to make a copy of a shorter document on a shorter length paper or a copy of a longer document on a longer length paper. A machine incorporating this capability is disclosed in Suzuki et al U.S. Pat. No. 3,977,666, for "Apparatus for Selectively Feeding Sheets from a Plurality of Cassettes".
While the Suzuki et al patent has the capability of selectively feeding sheets of different lengths from respective cassettes to a pair of synchronizing rolls, both of the cassettes have the same limited capacity. In a practical embodiment of the apparatus disclosed in the Suzuki et al patent, each cassette has a capacity of approximately 250 sheets. In the operative condition of each of the cassettes shown in the Suzuki et al patent, a spring biased member acting through an opening in the bottom of the cassette engages the forward portion of the sheet supporting plate to tilt the stack so as to bring a leading portion of the uppermost sheet into engagement with the feed roller of the machine. The principal defect of cassettes of the type shown in the Suzuki et al patent is their limited capacity. It is to be noted first that the capacity of one or both of the cassettes shown by Suzuki et al could not be increased merely by increasing the size of the cassette. The problem is to urge the stack of sheets so as to move the uppermost sheet against a pair of rollers with a substantially constant pressure or to move the stack so that the uppermost sheet is at a constant level with the rollers applying a constant pressure. In the system shown in Suzuki et al where only 250 sheets make up the stack, which is raised only a given fraction of the sheet length, one can use a linear spring to load the paper and as the paper is consumed the actual interface pressure between the top sheet and the rollers is more or less constant. This is because only part of the length of the stack is being raised so that the length of the sheets is not of major significance when only 250 sheets are involved. That is to say, the leading edge of the uppermost sheet is not displaced appreciably relative to the feed roll as the stack is depleted.
When one considers a large capacity magazine, such for example one holding 1000 sheets, it will readily be apparent that for geometric reasons it is not possible to provide an arrangement in which only the leading portion of the stack is moved upwardly against the rollers, owing to the fact that by the time the last sheet in the stack is reached, the slope of the paper will be so great as to produce a displacement of approximately one and one-half inches of its length relative to the position of the feeding rollers.
If as an alternative to raising the front of the stack an arrangement is considered in which the entire stack is raised for letter-size sheets, a weight of about four and one-half kilograms is involved, while for legal-size sheets about six kilograms must be raised. The difference of one and one-half kilograms between the letter and legal sizes means, if one employed a spring mechanism to raise the stack of paper which was the same for both letter and legal lengths of paper, a constant spring pressure independently of the number of sheets remaining in the stack and independently of the length of the sheets would be necessary. Such an arrangement is unfeasible. Thus there is a one and one-half kilogram discrepancy between the legal and letter paper sizes, together with the requirement, which I have found, of maintaining relatively constant pressure between the feed roller and the uppermost sheet of 300 grams, plus or minus 50 grams, before the mechanism will function properly. Faced with this problem, it has been suggested in the prior art that an electrical servomechanism be employed which will raise the stack of sheets to a constant level. More specifically, it has been proposed that a micro-switch be positioned to detect the level to energize a motor to actuate a lifting device when the paper drops down to below the predetermined level. In such an arrangement, the rollers themselves would be arranged to exert a certain pressure on the top of the sheet.
While the arrangement just described might operate satisfactorily, it incorporates a number of defects. It requires an electrical connection to the copier with which it is used. It is relatively expensive to construct and to operate. It is not readily adapted for use with existing machines.
While it is desirable that a user be able to make a copy of a short document on a relatively short sheet of paper or, alternatively, to make a copy of a relatively long original on a correspondingly long sheet of material, most users make a great many more copies of one length original than of another length. For example, in the usual commercial office, it is likely that many more copies of "letter" size documents be made than of "legal" size documents. On the other hand, in an office offering legal services, it is probable that a great many more "legal" size documents will be copied. It is thus desirable that the large capacity magazine have the capability of feeding sheets of both sizes. My large capacity magazine is able to be used with existing machines of the type shown in the Suzuki et al patent, as well as with an original machine.
I accomplish all of the above-outlined advantageous results without requiring an electrical connection to the existing machine. My large capacity cassette is relatively easily loaded without danger to the operator, which might occur if the paper supporting means is not properly controlled.